s these drivers can be operating at some fairly high voltages and currents it is a good idea to isolate them from the PC's parallel port, no point killing your computer if something goes wrong. For isolation I decided to use a very basic opto-isolator chip on each I/O line, I used the 4N25 opto-isolator in the default configuration right out of the datasheet I made a quick schematic of one input line, R1 is a current limiting resistor and R2 is a pull down resistor so pick the values accordingly. When wiring up the parallel port plug make sure you use the correct output pins, I used pins 2 - 9 as they are connected to the parallel ports data line and are non-inverting, just google the pinout for more information.